The invention relates to a method of manufacturing an ultrasound transducer, which method includes a step of forming a plate which is shaped as a disc of a composite piezoelectric material into a hollow spherical cap.
Ultrasound transducers are used notably in the medical field. There are numerous applications for such transducers.
There are ultrasound transducers which operate with low powers, that is, of the order of a few hundreds of milliwatts for imaging, as well as so-called ultrasound power transducers which are capable, for example, of destroying tumors within the human body by raising the temperature; such transducers are powered by currents of the order of from one watt to some hundreds of watts.
Generally speaking, ultrasound transducers enable a given quantity of energy to be focused in a small zone which is referred to as the focal spot which has the shape of an ellipsoid. The focal spot corresponds to the zone of convergence of the ultrasonic radiation produced by the transducer. Generally speaking, the radiation propagates in a direction normal to the surface whereto it is applied. The radiation together forms a so-termed ultrasound beam. Thus, the ultrasound beam is generally oriented in the direction which corresponds to the symmetry axis of the spherical cap. Each transducer has a focal distance which corresponds to the distance between the focal spot and the apex of the spherical cap of the transducer. The focal distance of a transducer is determined in principle by its geometry, that is, notably by the radius of curvature of the spherical cap. Thus, with each specific geometry of the transducer there is associated a given focal distance which is referred to as the geometrical focal distance of the transducer. The shape of the focal spot is more elongate in the direction of the major axis of the ellipsoid as the focal distance is larger.
The ultrasound transducers are made of a piezoelectric material, that is, a material which is deformed when subjected to electric current pulses. The deformations of the material produce radiation in the range of ultrasound vibrations, which radiation propagates in water or liquids and converges towards the focal spot in which it causes notably a rise of temperature. In the case of ultrasound power transducers this rise in temperature suffices to burn tissue of the human body, notably tumors which may be malignant or non-malignant.
In order to optimize the treatment, it is important that the focal distance of the transducer used is short. Indeed, this enables the dimensions of the focal spot to be reduced, thus enhancing the precision of the treatment. On the other hand, when a tumor is situated at a small distance from the external surface of the skin of the patient, the transducer can then be arranged in the vicinity of the external surface of the skin. This volume of the device is thus reduced and the coupling between the transducer and the skin facilitated, thus optimizing the penetration of the energy into the body of the patient. The focal distance can be varied slightly by means of an electronic device which enables dephasing of the vibrations for given zones of the transducer so as to increase or decrease the focal distance of the transducer in relation to its geometrical focal distance. Variation of the focal distance enables displacement of the focal spot in order to enlarge the zone of treatment, that is, without displacement of the transducer. The thickness of the spherical cap determines the frequency of the ultrasound radiation.
A method of realizing a transducer in the form of a spherical cap is already known from the publication xe2x80x9cFeasibility of Using Ultrasound Phased Arrays for MRI Monitored Non-Invasive Surgeryxe2x80x9d by Kullervo HYNYNEN et al., in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 43, No. 6, November 1996. This method utilizes a solid piezoelectric material in which a spherical cap of the desired dimensions is formed, notably a cap having the radius and thickness desired so as to obtain a given geometrical focal distance and radiation frequency. Subsequently, each spherical surface of the cap is covered with an electrode. When fed with an electric current, the electrodes enable the piezoelectric material to vibrate. Such a method is very expensive, because it requires a large quantity of piezoelectric material as well as exact machining operations. Moreover, the electronic device cannot be adapted so as to induce the dephasing of vibrations for given zones, so that the possibility of changing the focal distance of the transducer is lost.
It is also known to utilize composite piezoelectric materials which consist of a material which is formed by small elements of a piezoelectric material which are embedded in a matrix of an insulating material such as a polymer material. A disc of composite piezoelectric material is then formed, each surface of said disc being covered with an electrode which is realized by metallization in vacuum. When fed with an electric current, the electrodes enable the piezoelectric material to vibrate. The electrode on the rear surface of the disc consists of the juxtaposition of rings of a conductive material which are realized by photoengraving and chemical etching. The disc is thermodeformable because it consists of the polymer material. Thus, a hollow spherical cap is formed by deforming the disc under the influence of heat; this results in a shape having the desired radius of curvature.
However, the formation of the disc into a hollow spherical cap induces large mechanical stresses in the composite piezoelectric material, which stresses are larger as the radius of curvature of the ultrasound transducer is smaller.
During the operation of the transducer, the vibration at very high frequencies (of the order of from one to several MHz) of the piezoelectric elements also causes mechanical stresses inside the material.
The sum of the mechanical stresses must remain below the rupture strength limit of the composite piezoelectric material during use of the ultrasound transducer.
Thus, a method of this kind has its limitations when an ultrasound transducer having a small radius of curvature is to be realized. Indeed, for operation of an ultrasound transducer of a diameter of 100 mm and a thickness of approximately 1 mm at 1.5 MHz, the minimum radius of curvature which can be realized is of the order of 130 mm.
In order to solve these problems, the invention proposes a method of manufacturing an ultrasound transducer which includes a step of forming a plate which is shaped as a disc of a composite piezoelectric material into a hollow spherical cap as claimed in claim 1.
The invention also proposes an ultrasound transducer which is shaped as a hollow spherical cap manufactured by means of the above method.